1. Field of the Invention
Embodiments of the invention are in the field of improvement of transmission of audio information. Particular embodiments are in the field of improvement of transmission of tone signals according to RFC standard 2833.
2. Background of the Art
In communication networks, and especially in Voice over IP communication networks, compression methods—according to ITU standard G.729, for example—are used for transmitting speech and tones or tone signals, wherein the compression concepts used are best suited for speech transmission. When transmitting multi-frequency tones—known in the industry as DTMF dial tones—or fax/modem signaling tones, under certain circumstances the compression process can cause undesired changes in certain features of the digitized tones. In order to avoid such problems and facilitate tone recognition in the counterpart device, a standardized method according to RFC 2833 can be used to transmit fax/modem signaling tones, wherein this method is configured for transmitting tone signals that are used for signaling that creates a communication connection between Voice over IP terminals.
For example, when transmitting a fax/modem tone signal at 2100 Hz according to RFC 2833, the sending side recognizes and suppresses an incoming fax/modem tone signal or a digitized or analog tone signal. At the same time, a piece of tone information is generated in place of the recognized fax/modem tone signal and, in the case of a Voice over IP transmission, is inserted into special RTP (Real Time Protocol) data packets which are then sent over the Internet or over networks with Internet protocol. The tone information according to RFC 2833 contains data about the tone type, level, and duration of the fax/modem tone signal. On the receiving side, after the transmitted tone information is measured using a tone generator, the fax/modem tone signal is reproduced and inserted into the decoded signal from the RTP data packets. Because a time span of typically 20-30 ms is required for recognition of the transmitted fax/modem tone signals, it is not possible to suppress the portions or bit patterns of a fax/modem tone signal transmitted up to that point and transmit them further without an additional undesired delay. On the receiving side, when converting between the fax/modem tone signal produced in the tone generator and reinserted and the unsuppressed bit pattern of the fax/modem tone signal, there can be signal processes that a downstream tone detector may misinterpret as a phase reversal or phase shift or as amplitude modulation. These misinterpretations can occur especially in scenarios where a fax/modem tone signal is transmitted through multiple networks with Internet protocol or there are multiple transfers of the fax/modem tone signal according to RFC 2833. Misinterpretations often lead to significant problems or to complete failure when transmitting a fax/modem tone signal using data packets or RTP data packets.